#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <limits.h>
#include <map>
#include <queue>
#include <set>
#include <string.h>
#include <vector>

using namespace std;
typedef long long ll;

int N;
double dp[101][101][101];

double dfs(int r1, int r2, int r3) {
  if (dp[r1][r2][r3] >= 0) return dp[r1][r2][r3];

  dp[r1][r2][r3] = N * 1.0 / (r1 + r2 + r3);
  if (r1 > 0) dp[r1][r2][r3] += dfs(r1 - 1, r2, r3) * r1 / (r1 + r2 + r3);
  if (r2 > 0) dp[r1][r2][r3] += dfs(r1 + 1, r2 - 1, r3) * r2 / (r1 + r2 + r3);
  if (r3 > 0) dp[r1][r2][r3] += dfs(r1, r2 + 1, r3 - 1) * r3 / (r1 + r2 + r3);

  return dp[r1][r2][r3];
}

int main() {
  cin >> N;
  vector<int> A(N);

  for (int i = 0; i < N; ++i) {
    cin >> A[i];
  }

  int r1 = 0;
  int r2 = 0;
  int r3 = 0;
  for (int i = 0; i < N; ++i) {
    switch(A[i]) {
      case 0:
        r3++;
        break;
      case 1:
        r2++;
        break;
      case 2:
        r1++;
        break;
      default:
        // NOOP
        break;
    }
  }

  for (int i = 0; i <= N; ++i) {
    for (int j = 0; j <= N; ++j) {
      for (int k = 0; k <= N; ++k) {
        dp[i][j][k] = -1.0;
      }
    }
  }
  dp[0][0][0] = 0;
  cout << fixed << setprecision(10) << dfs(r1, r2, r3) << endl;

  return 0;
}